Piezoelectric Cantilever Microphone for Photoacoustic GAS Detector

Transcription

1 lntegmtedfrrmelectri~~.?001. Vol 35, pp Reprints available directly from the publither Photocopying permitted hy license only Q 2001 OPA (Oversear Puhlisherr As5ociatinn) N V, Published b) license under the Gordon and Breach Science Publishers imprint. Printed in Malaysia Piezoelectric Cantilever Microphone for Photoacoustic GAS Detector NICOLAS LEDERMANNa, JACEK BABOROWSKIa, ANDREAS SEIFERTa, BERT WILLINGa, STEPHANE HIBOUX, PAUL MURALTa, NAVA SETTERa and MARTIN FORSTERb aceramics Laboratoiy, Swiss Federal Institute of Technology CH-1015 Lausanne, Switzerland and bsiernens Building Technologies AG Cerberus Division, Ch-8708 Mannedod Switzerland (Received April 10, 2000) New micromachined pressure sensors based on PZT coated silicon cantilevers have been fabricated and integrated in a photoacoustic gas detector. PZT Sol-gel thin films texture and composition were optimized with respect to the transverse piezoelectric coefficient e3 I,f. A best value of -12 C/m2 was obtained with (100)/(001) textured thin films at the MPB composition. Optimum stress compensation between the different layers composing the cantilever has been studied in order to yield flat cantilevers. A high response of 150 mv/pa with a S/N of 700 at 1 Pa and 1 Hz bandwidth has been measured. The influence of the damping chamber under the cantilever is also reported. Keywords: MEMS; microphone; cantilever; piezoelectric INTRODUCTION Photoacoustic gas sensing is a proven technique used in gas analysis [I]. It is based on the infrared absorption at a specific wavelength which is [1907]/177

2 178/[ 1908 J NICOLAS LEDERMANN et nl characteristic for the gas to be detected (for example, 4.25 pm tor COz, 3.55 pm for CH4). In this application, the gas is heated by a modulated IR light source (hot filament) and the presrurc amplitude due to the heat absorbed is then measured by a pressure sensor (see FIGURE I). FIGURE I. Schematic of the photoacoustic gas sensor Thc typical pressure amplirude IS 100 mpa and corresponds to a concentratioii of the analyzed gas of approximately 5000 ppni [ZJ. Smcc the ik light modula- tion takes placc at a frequency lower than 100 HL, there is a need for dedicated microphones with a high sensitivity at low frequencies. Up to now, only prcs- sure sensors based on meinbianes have bccn used [I] However, cantilevers nith picmeiectnc thin films are basic structures for a number of applicauons, such as accelcroineteis [3], audio microphone & microspeakers [41 and AFM probes [S] Due to their high intnnsic responsivity (more than two times higher than membrane of the same axed and thickncsr), they arc good candrdates for high performance micropboncs The slit around the cantilever has to be made very nanow in order to reduce the air conductance to the backside Widths of 3 to I0 pm 5hould be achieved for a 10- cut-off ftcqucncy (10-20 HL)

3 PIEZOELECTRIC CANTILEVER MICROPHONE _.. [1909]/179 COKCEPT AND DEVICE PHYSICS Thc cantilever concept for an audio microphone has been proposed by White & al. in 199G 141. A ZnO coated cantilever has been used either as microphone or microspeakcr in the audio range of frequency (> 700 Hz). For photoacoustic gas detection, a high sensitivity at a low frequency (10-20 Hz) is required. This limitation comes from the thernial inertia of the microinfrared light source (inmima1 efficiency around 10 Hz). for this, thin and perfectly flat cantilevers with a slit as narrow as 3 to 10 pn are required.? he response of the microphone (see FIGURE 2) due to an AC pressure excitation can be derived as follows. FIGURE 2. Silicon cantilever under an AC pressure amplitude The in-plane strain in the piezoelectric thin film along the 1-axis, in the limit of a thin PZT film (t, c< t), is given by cquation (1). Wherc t IS the cantilever thickness. Es, thc silicon Young s modulus, L the length of the cantilever and P the ampfitudc of thc pressure ewiation.

4 180/[ NICOLAS LEDERMANN et a1 Due to the transverse piezoelectric effect, this strain creates a D-field along the 3-axis. Where D3(x) is the charge density, e31,f the transverse piezoelectric coefficient and v the silicon Poisson coefficient. The charge is then given by (2) integrated between 0 and L/2 and multiplied by the width wei of the top electrode. With a e31,f of -12 C/m2, L = wel = 2 mm and t = 15 pm, a response of about 4 pcpa can be expected. The current response under a pressure excitation Peiwt is then given by: L Optimization studies on PbZrxTil-x03 thin films have shown that (100)/(001) textured films exhibit the highest transverse piezoelectric coefficient (-12 C/m2) at the morphotropic phase boundary 53/47 composition (see FIGURE 3). In order to fabricate flat structures, the stress in the cantilever needs to be

5 PIEZOELECTRIC CANTILEVER MICROPHONE. [1911]/18l compensated. The tensile stress of the PZT thin film ( MPa after polarisation) and the platinum bottom electrode (100 nm MPa) have been compensated by 650 nm of thermal silicon oxide (-300 h4pa) (see FIG- URE 3) Cornpositon [ZrY([ZrjtlT~]) FIGURE 3. Transverse piezoelectric coefficient and thin film stress of(100)/(001) textured PZT thin film. MICROFABIUCATION AND PROCESSING The microfabrication process of the cantilevers is made within six main steps. First, a 1 pm sol-ge1(100)1(001) PZT thin film is deposited on a platinum bot- tom electrode with a 10 nm PbTiO, (100) seeding layer [6]. Then, a 350 nm Sic thin film is deposited on the backside. It will be used as masking layer dur- ing the silicon bulk micromachining. Then, backside windows are opened through SiC/SiO, by dry and wet etching. Au/Cr top electrodes are then evapo- rated and structured by lift-off. The slit around the cantilever is then structured by dry ion gun) through PZT/Pt/SiO2 [7]. The silicon substrate is then etched in KOH down to the required cantilever thickness and, finally,

6 I82/[ NICOLAS LEDERMANN et a/. the cantilevers are released by Si dry etching from the front side. The final microphone is shown in FIGURE 4. The measurements of the transverse piezoelectric coefficient on test samples inscrtcd on the device wafer have given a value of C/m2. Obviously, the microfabrication process has only little influence on the piezoelec1ric properties. F IGURE 4. (a) 2x2 mm PZT/Si cantilever microphone and (b) SEM crosssection of a 3 pm slit. RESULTS AND DISCUSSION The pressure responsc of the microphone is measured in a sealed box where a pressure excitation is created by a loudspeaker. The signal is then compared to a rcfcrence microphone. FIGURE 5 shows the pressure response of the microphone C % of the maximum sensitivity is already achieved at 20 Flz. The maximum value tends to 150 mv/ya. A noise level of 210 pvmz? has been measured. This means that the noise equivalent pressure amounts to 1.4 mpa at a bandwidth of 1 Hi... With a cantilever thickness of 17 pm, the measured response corresponds to about 608 of the theoretical value. As the microphone needs to be insolated from the external environnement, the volume under the cantilever (so-called damping chamber) is of a great influence on the frequency response. At constant frequency, the sensitivity will increase as the

7 PIEZOELECTRIC CANTILEVER MICROPHONE... [ I91 3]/183 damping volume decreases. As the conductance of the slit IS constant, this effect is simply due to the time needed to equal the pressure on both sides of the cantilever. This time increases when increasing the vohme of the chamber e, d $ FIGURE 5. Microphone response as a function of the excitation frequency FIGURE 6 shows the pressure response of a microphone as a function of the damping volume. In that case, a volume of a least lo00 mm3 is required in order to obtain a sufficient response. 16 FIGURE 6. Influence of the air damping volume on the frequency response (microphone fabricated with non-optimal good PZT, 45/55, (111) textured)

8 184/[ NICOLAS LEDERMANN et al. CONCLUSION Pressure sensors based on a PZT/Silicon cantilever have been fabricated by micromachining techniques and integrated into a new photoacoustic gas sen- sor. (100)/(001) textured PZT thin films with a MPB composition and exhibit- ing a high transverse piezoelectric coefficient of ejl,f = -12 C/m2 have been sucessfully integrated into the devices. Perfectly flat cantilevers have been obtained by compensating the tensile stress of the PZT and the platinum bot- tom electrode with 650 nm of silicon thermal oxide. A response of 150 mv/pa for a 2x2 mm cantilever with a noise equivalent pressure of 1.4 mpa at a band- width of 1 Hz has been measured. The measured response corresponds to approximately 60% of the value given by the model. The difference can be explained by the influence of the air damping. ACKNOWLEDGMENTS This work was supported by the Swiss Priority Program on Micro- and Nano- Systems Technology (MINAST) References [I1 L.B. Kreuzer, Journal of Applied Physics, vol42 n"7, 2934 (1971). [2] M.D. Weber, Thesis no 2075, (2000) EPFL, Switzerland. [3] J. Baborowski, S. Hediger, C. Wuetrich and P. Muralt Ferroelectrics 224, (1 999). [4] S.S. Lee, R.P. Ried and R.M. White, J. of microelectromechanical systems, vol 5 n"4, (1996). [5] C. Lee, T. Itoh, T. Suga, Sensors and Actuators A 72, (1999). [6] S. Hiboux and P. Muralt, Ferroelectrics ~01224, (1999). [7] J. Baborowski, P. Muralt, N. Ledermann, A. Seifert and N. Setter Integrated Ferroelectrics 2000, in press.